1. Field of the Invention
The present invention relates generally to a hydraulic shock absorber for an automotive suspension system. More particularly, the invention relates to a hydraulic shock absorber which is capable of varying damping characteristics during each vibration cycle including bounding and rebounding stroke. The invention further relates to control of suspension characteristics for suppressing vehicular attitude change.
2. Description of the Background Art
One of the typical variable damping force suspension system has been disclosed in Japanese Patent First (unexamined) Publication (Tokkai) Showa No. 61-85210. In the disclosed system, a piezoelectric element is disposed in each shock absorber in each suspension system for detecting variation of fluid pressure in the shock absorber. A control unit is provided for receiving an input indicative of the fluid pressure provided from the piezoelectric element. The control unit outputs a controlled voltage to the piezoelectric element for switching operation mode of the shock absorber at least between SOFT mode in which smaller damping force is to be generated in response to vibration input and HARD mode in which greater damping force is to be generated in response to vibration input. frequency input vibration which induces attitude change of the vehicle body to switch the operational mode of the shock absorber into HARD mode for a given period of time. While the shock absorber is maintained at the HARD mode, the piezoelectric element maintains operation as an actuator for maintaining HARD mode operation of the shock absorber. Therefore, while it is active as the actuator, the piezoelectric element can not monitor fluid pressure.
In the modern technology of suspension control, it has been considered that varying of damping characteristics of shock absorber between a piston compression stroke in response to bounding motion between the vehicle body and a road wheel and a piston expansion stroke in response to rebounding motion between the vehicle body and the road wheel in order to obtain better vibration stabilizing performance. Therefore, it is desirable to adjust the damping characteristics of the shock absorber depending upon the mode of piston action. In order to realize this, it is essential to detect the piston action mode on the basis of variation of the fluid pressure in the shock absorber. However, as set forth above, since the piezoelectric element is held in operative as the fluid pressure sensing element while the shock absorber is maintained at the HARD mode.
This may cause problem in damping shocks. For example, when the damping characteristics in the HARD mode is set to generate relatively great damping force in response to vibration input, damping force generated in response to piston compression mode action can amplify the input vibration. This tendency may be significant for the second and subsequent vibration cycles. This clearly degrades vibration stabilizing performance of the vehicle to provide rough ride feeling.
In addition, in the prior proposed suspension system operates substantially in passive manner to detect the input vibration based on variation of fluid pressure in the shock absorber. Namely, until the vehicle driving condition is changed to require switching of suspension characteristics, the suspension system will never operate to switch suspension characteristics. Therefore, when a criterion of the fluid pressure to switch the suspension mode from SOFT mode to HARD mode is set at relatively high value in order to provide better riding comfort, switch from SOFT mode to HARD mode tends to delay to cause bottoming due to low response characteristics particularly at the initial stage of switching of suspension mode.
On the other hand, in the modern automotive suspension systems, there have been proposed various suspension control systems which can control suspension characteristics for suppressing vehicular body attitude change and whereby achieves high level riding comfort and driving stability. Some of the prior proposed suspension control systems utilize variable damping force shock absorber for adjusting suspension characteristics and whereby suppressing attitude change. The vehicular attitude change is detected by means of a sensor, such as vehicular height sensor, acceleration sensor and so forth in order to perform attitude change suppressive suspension control, such as anti-rolling, anti-pitching control or so forth.
Such prior proposed suspension control system still contains various problems and thus is not satisfactory in achieving satisfactory level of riding comfort and driving stability.